The average fuel economy for all vehicles sold last year climbed by 1.3 miles per gallon, or 6%, to a record high of 23.8 mpg, according to University of Michigan researchers Michael Sivak and Brandon Schoettle.

The fuel economy of cars people purchased over the last year increased even though gas prices have dipped slightly. AAA said the average price of a gallon of regular gas stood at $3.297 as of Monday, compared with $3.371 a year ago.

Since 2008, the first full year of monitoring by Sivak and Schoettle, fuel economy has risen 14% -- from 20.9 mpg.

On a monthly basis, fuel economy in December was 23.9 mpg, a slight drop from 24.1 mpg in November.

December was a huge month for sales of gas-thirsty trucks, driven by big discounts, as Ford and Chevrolet battled for market share, and a worry by business that Congress would end certain tax advantages for truck purchases as part of its "fiscal cliff" negotiations. (The tax benefits ultimately survived the political fray.)